The present invention relates generally to reducing chopping noise, also referred to as ripple noise, in chopper stabilized operational amplifiers, and more particularly to using switched capacitor notch filters to reduce ripple noise.
It is highly desirable that integrated circuit operational amplifiers have low offset voltage, low noise, low-offset drift, and good signal stability. Chopper stabilization and auto-zeroing are two common techniques that have been widely used to reduce amplifier offset voltage and drift. (For example, conventional chopper stabilization would typically reduce a 5 millivolt offset voltage to roughly 5 microvolts.) Modern chopper-stabilized operational amplifiers and autozero operational amplifiers have significantly reduced, or even essentially eliminated, the amount switching noise therein compared to previous designs. However, the improved design techniques used in modern chopper-stabilized operational amplifiers and auto-zero operational amplifiers result in trade-offs between input referred noise and quiescent supply current (Iq). The inherent trade offs between basic chopper-stabilized amplifiers and autozero amplifiers are well known. While the auto-zeroing method provides low ripple noise at the amplifier output, its in-band noise is high due to aliasing or noise folding. On the other hand, the chopper stabilization technique presents lower in-band noise due to absence of noise folding, but its output ripple noise is relatively higher. Basic chopper-stabilized amplifiers maintain the broadband noise characteristics of their input stages, but “shift” their input offset voltages up to the chopping frequency, creating large ripple voltages at the amplifier outputs. Although basic autozero amplifiers do not shift their input offset to their autozero frequency like chopper-stabilized amplifiers, they suffer from aliasing or folding back of their broadband noise spectrums during their zeroing cycles, which increases the overall input referred noise of the amplifiers.
It can be shown that for an ideal input stage, the square of the input referred noise is inversely proportional to the quiescent supply current Iq of the amplifier, which causes the basic autozero amplifiers to have significantly increased quiescent supply current Iq in order to achieve the desired noise levels, including the aliasing or noise folding. This makes it very desirable to use chopper-stabilized amplifiers in micropower applications and to find a way of solving the basic limitation of ripple noise at the chopping frequency.
FIG. 1 shows a conventional basic three-stage amplifier 1A with multipath nested Miller compensation. This circuit configuration can be thought of as including a three-stage high gain signal path including three sequentially coupled stages 2, 3 and 4 having transconductances of gm1, gm2, and gm3, respectively, coupled in parallel with a wider bandwidth two stage signal path including two sequentially coupled stages 5 and 4 having transconductances of gm4 and gm3, respectively. The amount of DC precision of the operational amplifier shown in FIG. 1 is determined by the input stage 2 in the three-stage high gain signal path, while the high frequency response and phase margin are dominated by the two-stage signal path. Proper selection of the transconductances and the compensation capacitances results in the operational amplifier having the bandwidth and settling characteristics of a two-stage Miller compensated operational amplifier with a minimal increase in quiescent supply current Iq being required to achieve a good GBW/Iq (i.e., gain-bandwidth/Iq) ratio.
FIG. 2A shows the basic operational amplifier configuration of FIG. 1 further including basic chopper stabilization circuitry added before and after the input stage 2 in the high gain three-stage DC signal path. The chopper stabilization has the advantage of substantially reducing offset voltage, offset voltage drift with respect to temperature, and flicker noise, but has the disadvantage of shifting the offset voltage of the input stage 2 to the chopping frequency fs and thereby producing a large ripple voltage component in the amplifier output Vout.
There is an unmet need for a chopper-stabilized amplifier having extremely low output ripple noise.
There also is an unmet need for a chopper-stabilized operational amplifier having extremely low output ripple noise and a very low offset voltage.